Typically, milk and milk-related products are heat treated in order to inactivate undesirable enzymes and destroy pathogenic and spoilage microorganisms. The heating process may additionally cause physical and chemical changes (protein denaturation, browning, etc.), which positively or negatively affects the sensory characteristics and nutritional value of the products. Milk-related products may be treated by a range of processes which differ in the severity of the heat treatment.
The three general types of heat treatment (from mild to severe) are thermization, pasteurisation and sterilization. Thermization is a mild heat treatment (typically 57-68° C. for 15 sec.) sufficient to destroy gram-negative psychotropic vegetative microorganisms and increase the refrigerated shelf-life. Pasteurisation (typically 72° C. for 15 sec.) destroys most of the vegetative pathogenic organisms (bacteria, yeasts, and moulds), which may cause food poisoning. Sterilization is the most severe heat treatment (typically +121° C. for 3 min.) and destroys all microorganisms (vegetative and spores) or renders them incapable of further growth.
Ultra high temperature (UHT) processing is well-known in the prior art as a continuous flow process, where the milk is heated in excess of 135° C., held for approx. 4 sec, rapidly cooled, and aseptically packaged. UHT milk undergoes fewer chemical reactions than sterilized milk, resulting in a product that is whiter, tastes less caramelised, has reduced whey protein denaturation, and reduced loss of heat-sensitive vitamins.
Even so, the development of off-flavours, especially stale or oxidized flavour, during storage, is an important factor limiting the acceptability of UHT milk. This off-flavour development is associated with chemical reactions and changes (e.g. Maillard reaction and browning) that occur during processing and that continue during subsequent storage. The perceived poor flavour of UHT milk is one of the common problems faced by the dairy industry, since it is a strong barrier to consumer acceptance of UHT milk. For a review, see Al-Attabi, Z., D'Arcy, B. R. and Deeth, H. C. (2009) Critical Reviews in Food Science and Nutrition, 49: 28-47.
U.S. Pat. No. 4,053,644 discloses a process of treating fluid milk having a “cooked flavour”.
The process comprises substantially removing the cooked flavour by contacting said fluid milk with immobilized sulfhydryl oxidase enzyme. However, sulfhydryl oxidase is not easily available and is used mainly in baking industry, thus unsuited for dairy products.
WO 99/09143 discloses the use of one or more oxidoreductases, in particular laccase, for the reduction of malodour in various products such as oral care products, hygiene products. Also mentioned are products for removing burned flavour from foods such as milk. However, laccase (DeniLite®; www.novozymes.com) has its main application in the textile industry and is not optimised for dairy products.
Consequently, there is a need for improved processes for reducing off-flavours in e.g. UHT milk, which involves the use of enzymes that are easily available and suitable for dairy products.
Lactose oxidase, a carbohydrate oxidase with high specificity of oxidizing the disaccharide lactose to lactobionic acid (LBA) has been disclosed by Ahmad et al. (2004) Regul. Toxicol. Pharmacol. 39(3): 256-270. LBA occurs naturally in certain fermented milks (Kiryu, T. et al. (2009) J. Dairy Sci. 92: 25-34). LBA is also used in specific applications within the food industry and may be used to increase cheese yield. Lactose oxidase is included in the product LactoYIELD® (www.chr-hansen.com).